Production of polyethylene terephthalate using an antimony compound as catalyst



PRODUCTION OF POLYETHYLENE TEREPH THALATE USING AN ANTIMONY COMPOUND AS CATALYST 7 Harry R. Billica, Seaford, Del., and Jonathan Turner Carriel, Pittsburgh, Pa.,. ass'ignors to E. I. du Pont de' Nemours and Company, Wilmington, DeL, a corporation of Delaware I No Drawing. Application January 3, 1952, Serial N0. 264,852

Claims. (01. mp7s nited States Patent ger from 2 to 10 inclusive, is fully disclosed in U. S. P.

2,465,319 to Whinfield and Dickson. From a commercial standpoint, one of the most attractive polymers of this class is polyethylene terephthalate; and the most promising process for its production comprises carrying out an ester interchange between ethylene glycol and dimethyl terephthalate to form bis-2-hydroxy-ethyl Vterephthalate monomer which is polymerized to polyethylene terephthalate under reduced pressure and at elevated temperatures.

The above patent discloses numerous compounds which are suitable in varying degree as catalysts in the ester interchange reaction. Compounds disclosed include lithium, sodium, potassium, calcium, magnesium, zinc, cadmium, aluminum, manganese, iron, nickel and antimony. The alkali metals, the alkaline earth metals and magnesium are conveniently used in the form of alcohol-ates, or in the form of carbonates orborates. Magnesium may also be used in the form of its oxide. However, in no instance do these suggested materials satisfactorily catalyze both the ester interchange and the polymerization cycles; and, where catalysis of the complete reaction is desired, these catalytic materials are not satisfactory.

An object of the present invention, therefore, is to provide a catalyst system for accelerating and controlling the reaction between ethylene glycol and dimethyl terephthalate, and the subsequent polymerization of the resulting product. A further object is to provide a catalyst system for accelerating both esterinterchange and polymerization, and, at the same time, prevent color degradation of the polymer. These and other objects will more clearly appear hereinafter.

The above objects are accomplished according to the present invention by carrying out the ester interchange between ethylene glycol and dimethyl terephthalate and subsequent polymerization of the resulting bis2-hydroxy ethyl terephthalate under super polyester-forming condi-.

tions in the presence of catalytic amounts of a calcium compound and an antimony compound.

Any compound containing calcium is useful for purposes of this invention. The following examples are representative of the diverse types of suitable calcium compounds: calcium hydride, calcium oxide, calcium peroxide, calcium hydroxide, calcium carbide, calcium cyanamide, calcium salts of inorganic acids, such as calcium bromide, calcium borate, calcium tungsta'te, calcium titanate, calcium silicate, andcalcium'salts oforganic acids, such as calcium formate, calcium acetate, calcium hydroxyacetate, and calcium stearate. The colorless or Patented Mar. 27, 19 56 ice White compounds which dissolve in the reaction mixture, for example, calcium acetate, and calcium hydroxide are particularly preferred. In comparison with previously proposed catalysts (e. g., litharge), the use of calcium compounds permits lower ester interchange reaction tem peratures; and the color of the intermediate and final product is correspondingly improved. The calcium compound should be used in amounts designed to provide from 0.00035 to 0.011 of an atom of calcium per mol of dimethyl terephthalate (0.033% to 1% by weight of hydrous calcium acetate based on dimethyl terephthalate), the preferred range being 0.001 to 0.003 .of an'atom of calcium per mol of dimethyl terephthalate. Lesser amounts of the catalysts, are decreasingly effective in promoting a rapid reaction; and larger amounts do not in crease the reaction rate proportionately, if. at all, and may even give poor color.

The calcium compounds. appear to primarily catalyze the ester interchange reaction; whereas, the antimony compounds function mainly as polymerization catalysts, and have little or no effect on the ester exchange reaction. Consequently, the latter may be added together with the calcium compounds at the beginning of the reaction; or they may be added after the ester exchange is complete. As in the case of calcium compounds, any compound of antimony-is suitable for purposes of this invention. The preferred antimony compounds are the colorless or white compounds in which antimony exhibits the valence of three; for example, antimony trioxide,

antimonous oxychloride, antimony trifluoride, sodium antimony hydroxy acetate, and antimonyl potassium tartrate. The antimony compound should be used in amounts designed to provide from 0.0003 to 0.0013 of an atom of antimony per mol of dimethyl terephthalate (0.023 to 0.10% by Weight of antimony trioxide, based.

on dimethyl terephthalate). Again, lesser amounts of the catalysts are decreasingly effective; and larger amounts do not increase the reaction rate proportionately, if at all,

, and may even give lower viscosity and/or poorer color.

Of the antimony compounds mentioned, antimony trioxide and antimonyl potassium tartrate (tartar emetic) are preferred. The latter is soluble'in cold glycol and is particularly useablein continuouspolymerization, especially in the final stages when the polymer melt is very viscous and agitation is necessarily reduced.

The following examples of preferred embodiments willserve to illustrate the principles and practice of this;

invention. Parts are by Weight unless otherwise indicated:

EXAMPLE I This example illustrates the use of various combina lowing manner: fifty parts of dimethyl terephthalate andi 55 parts of ethylene glycol and the indicated amount of catalysts were heated in a three-neck .flask,'equipped with a stirrer and a condenser,

until the evolution of methanol stopped. The temperature of the reaction mixture/was -fixed at any given time by its boiling column in the table records theatemperature range-- dur-- ing the evolution of methanol, the first temperature being that at which the evolution. of methanol started, and the" final temperature being that of the reaction mixture-when the evolution of methanotceased. The time for commapoint at atmospheric pressure. The third polyethylene terephthalate from di- (DMT) and ethylene glycol. The" at atmospheric pressure tion of the initial exchange reaction is recorded in the fourth column of the table as the time for evolution of methanol. At the end of methanol evolution, 15 to 20 parts of the liquid in the flask were placed in a polypolyethylene terephthalate having an intrinsic viscosity of 0166. The color of this polymer was a medium amber, much darker than that of the polymer of the previous example.

merization tube and heated at 275 C. under a vacuum EXAMPLE. of 0.5 to 1.0 mm. of mercury for two hours and, in some I cases, for seven hours. The material in the polymeriza- The fbllowing ingredientsrwere introduced into a Staintion'tube was agitated continually by bubbling nitrogen less Steel batch Still q pp with a Slim?! and 3 C011- through it. The color of the polymer melt was deterdenser; mined by comparison with arbitrary color standards pre- I pared by dissolving known amounts of Du Pont Pon- 3 3 2 2 replh t-halate g yco 155 lbs. iammg Qatechu dye m a fixed amount of Water The Calcium acetate (containing one mol of H20 per mol of color ratings are as follows: calcium acetate), 0,312 lb. (0.0014 atom of calcium 0=water. per mol of DMT). 1=0.00025 gram of dye per 100 ml. of solution. Potassium antimonyl tartrate, 0.12 lb. (0.0003 atom of 2=twice as much dye as 1. antimony per mole of DMT). ii gg p i z z i ig g z The ester-exchanger'eaction began at 121 C. and rey p quired 4.0 hours to complete the removal of methanol. The intrinsic viscosity of the polymer plug, as noted in The resulting bis-2-hydroxy-ethyl terephthalate was transthe last column of the table, was determined in dilute ferred to a; 250-poundstainlesssteel autoclave equipped solutions of the polymer in Fomal, which comprises 58.8 with a stainless steel stirrer. The autoclave was evacparts by weight of phenol and 41.2 parts by weight of uated gradually through an ice trap and a Dry Ice trap trichlorophenol. while the temperature of the batch was raised to about Table I Temp. Atoms of range Time to Hrs. at am: has. as; [m of DMT retgcgon n01, hrs. uum

0.005 part litharge 186-220 3.1 3 2 3333 3335333%?352553: 33333 N 3 3 13. 31313335333333.313l3ig3i: 3383i n 2 .44 stanzas-teases: use w 2 W a 0.065 part calcium acetate 0.0014 1]) 2 1+ .42 0.025 part potassium antimonyl 164221 1. a

.tarttate 0. 0003 7 a .75 0.065 part calcium acetate 0. 0014 0.035 part sodium antimony }172213 1.1 7 3+ .73

hydroxy acetate 0.0003

EXAMPLE II 45 275 C. When the pressure inthe autoclave was reduced The following ingredients were introduced into a stainto p of mercury lfolymenzanon less steel batch still equipped with a stirrer and a con- Gin-{ed 3 9 hours at t in vacuum Therm" denser: trinsic viscosity of the result ng polymer was 0.58 as measured in Fomal. The color of the polymer melt was Dimethyl terephthalate, 58 lbs. very pale green." f Ethylene glycol, 40 lbs. EXAMPLE e Calcium hydride, 0.0174 lbs. (0.0014 atom of calcium L 1 per mol of DMT). The procedure ofExample IV was repeated except that Antimony trioxide, 0.0174 lbs. (0.0004 atom of antimony the potassium antimonyl tartrate was replaced by 0.072 per mol of DMT). lb. of antimony trioxide (0.0004atom of antimony per The ester-exchange'reaction started at 118 C., and 3.5 molcof r i reacuon started at' hours were required to remove the methanol formed. 148 i s only q for complete.remova.l of The resulting material, principally bis-Z-hydroxy-ethyl methanol The Polymerization at 3111 F p l terephthalate, was transferred to a 50-pound stainless hours to reach a polymer intrmslcl vlscoslty i steel autoclave equipped with a stainless steel stirrer and The 60101" of i Polymer me 3 was a so very pa heated to about 275 C. 1

The autoclave was evacuated gradually through an ice It IS to undelistood that the foregoing i es trap and a Dry {Ce trap; and when the pressure in the am are merely illustrat ve and that the present invention toclavewas reduced to about 0.5 mm. of mercury, the broadly comprises conducting an ester lnterchange and polymerization was carried out for 5.1 hours at this full Subsequent polymafnzanon resultmg f' vacuum. Theintrinsic viscosity of the resulting polyeihyi iF i i P condl' met as measured in Fomal was Q66 and the Polymer had tions in the presence of catalytic amounts of calcium coma very pale green tint. 1 pounds and antimony compounds.

. The initial condensation may be regarded essentially EXAMPLEHI as a simple ester interchange and may be conveniently \The'procedm-e f Example II was repeated except that 70 carried out atatmospheri pressure and at a temperature the catalyst employed was 0.0058 lb. of litharge '(0.01% range between -260 C. and, preferably, between byweight of dimethyl terephthalate). The ester-exchange PTha macho may also be F out reaction: began at C. and required 45 hours f under pressures above and below atmospheric pressure, complete removal of methanol. The subsequent poly- I if-desired. v merizationrequired 9.3 hours at full vacuum to obtain Polymerization may be effected in either the liquid (i. e., melt) or solid phase. In the liquid phase, the reaction must be carried out at reduced pressure in the vicinity of 0.05-20 mm. of Hg with a range 0.05-5.0 mm. Hg preferred for optimum results. This reduced pressure is necessary to remove free ethylene glycol which emerges from the polymer as a result of the condensation reaction, since the reaction mixture is very viscous. A temperature between 230290 C. and, preferably, between 260275 C., should be maintained during the polymerization step.

The catalyst system of the present invention is particularly effective for polymerization of relatively large batches of monomer. Furthermore, the catalyst system is highly useful in accelerating ester interchange and polymerization when carried out continuously. By comparing the color grade of the polymers produced in Examples II and III, it is obvious that the color of the polymer prepared in the presence of litharge alone would generally not be acceptable for conversion into filaments and, particularly, for conversion into film. On the other hand, by using the catalyst system of the present invention, polymer of acceptable color for commercial use can be prepared in a shorter time than with litharge alone.

While dimethyl terephthalate is the preferred ester monomer, and the present invention has been described with particular reference to this starting material, the catalyst system of this invention also eflfectively catalyzes the ester interchange between ethylene glycol and terephthalate esters of saturated aliphatic monohydric alcohols containing up to and including 4 carbon atoms, e. g., diethyl, dipropyl and di-isobutyl terephthalates.

As many different embodiments may be made without departing from the scope and spirit of my invention, it is to he understood that the invention is in no way restricted save as set forth in the appended claims.

We claim:

1. In the process for producing filamentand filmforming polyethylene terephthalate wherein ethylene glycol is reacted under ester interchange conditions with 40 an alkyl ester of terephthalic acid and a saturated aliphatic monohydric alcohol containing from 1 to 4 carbon atoms, and the resulting glycol terephthalate is polymerized, the improvement which comprises carrying out the ester interchange reaction in the presence of a catalyst comprising essentially at least one calcium compound and carrying out the polymerization reaction in the presence of a catalyst consisting of at least one calcium compound and at least one antimony compound.

2. The process of claim 1 wherein the catalyst system consists of a calcium compound providing from 0.00035 to 0.011 of an atom of calcium per mol of the alkyl ester of terephthalic acid, and an antimony compound providing from 0.0003 to 0.0013 of an atom of antimony per mol of alkly ester of terephthalic acid.

3. The process of claim 1 wherein the calcium compound and the antimony compound are both incorporated in the initial reaction mixture of the monomeric ester and alcohol reactants.

4. In the process for producing filamentand filmforming polyethylene terephthalate wherein ethylene glycol is reacted under ester interchange conditions with dimethyl terephthalate, and the resulting glycol terephthalate is polymerized, the irnprovment which comprises carrying out the ester interchange reaction in the presence of a catalyst comprising essentially a calcium compound providing from 0.00035 to 0.011 of an atom per mol of dimethyl terephthalate and carrying out the polymerization reaction in the presence of a catalyst system consisting of said calcium compound and an antimony compound providing from 0.0003 to 0.0013 of an atom of antimony per mol of alkyl ester of terephthalic acid.

5. The process of claim 4 wherein the catalyst system consists of a calcium compound providing from 0.001 to 0.003 of an atom of calcium per mol of dimethyl terephthalate, and an antimony compound providing from 0.0003 to 0.0013 of an atom of antimony per mol of the dimethyl terephthalate.

References Cited in the file of this patent UNITED STATES PATENTS 2,465,319 Winfield et a1 Mar. 22, 1949 2,534,028 Izard Dec. 12, 1950 2,647,885 Billica Aug. 4, 1953 

1. IN THE PROCESS FOR PRODUCING FILAMENT- AND FILMFORMING POLYETHLENE TERPHTHALATE WHEREIN ETHYLENE GLYCOL IS REACTED UNDER ESTER INTERCHANGE CONDITIONS WITH AN ALKYL ESTER OF TERPHTHALIC ACID AND A SATURATED ALIPHATIC MONOHYDRIC ALCOHOL CONTAINING FROM 1 TO 4 CARBON ATOMS, AND THE RESULTING GLYCOL TEREPHTHALATE IS POLYMERIZED, THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE ESTER INTERCHANGE REACTION IN THE PRESENCE OF A CATALYST COMPRISING ESSENTIALLY AT LEAST ONE CALCIUM COMPOUND AND CARRYING OUT THE POLYMERIZATION REACTION IN THE PRESENCE OF A CATALYST CONSISTING OF AT LEAST ONE CALCIUM COMPOUND AND AT LEAST ONE ANTIMONY COMPOUND. 